The incorporation of nano particles into polymers is a general materials design approach that is employed in all areas of materials science. The framework is attractive because it enables the creation of materials with new or improved properties by mixing multiple constituents and exploiting synergistic effects. Nanocomposites involving electrically (semi)conducting conjugated polymers are of interest because their (opto)electronic properties can be widely manipulated through combination of different types of materials. Nanocomposites which comprise high-aspect ratio conjugated polymer particles, i.e. nanofibers or rods, can offer a number of special features, including percolation at low concentration, anisotropic properties, and high internal surface area, which is important for photovoltaic applications. However, few broadly applicable approaches are known that allow the fabrication of well-individualized conjugated polymer nanofibers or rods and nanocomposites thereof. The experimental research program supported by this grant explores a new, broadly useful framework for the fabrication of such materials. The approach relies on the use of cellulose nanofibers, which can be isolated from several biological sources, as a high-aspect ratio nano-scale scaffold. These cellulose nanofibers are decorated with several strategically chosen (semi)conducting polymers and electroactive small molecules through either adsorption of pre-formed polymers, reaction with small molecules, or in-situ polymerization of macromolecules. Electrically conducting nanofibers thus prepared are processed into nanocomposites with 'passive' carrier polymers with the objective to create materials that display low percolation threshold, that is, high electrical conductivity at low filler content. Semiconducting nanofibers prepared by this framework are compounded with other conjugated polymers with the objective to create co-continuous materials that are comprised of two different semiconductors. Such semiconducting nanocomposites with high internal surface areas are of interest in various applications, including photovoltaic devices, which are explored in this study as one important example. One method for the processing of the new materials is a template approach. This recently developed fabrication scheme is based on the formation of a three-dimensional network, which is assembled through the gelation of a dispersion of originally well-individualized nanofibers. The gelled nanofiber template is then imbibed with a polymer of choice, before the nanocomposite is dried and shaped. One objective of this program is to demonstrate that the template approach is a broadly applicable processing method.
NON-TECHNICAL SUMMARY: Polymer nanocomposites comprising one-dimensional (semi)conducting conjugated nanofillers are of broad interest in applications that range from inexpensive conducting plastics to high efficiency polymer solar cells. Unfortunately, these materials are very challenging to produce. The experimental study supported by this grant explores a new framework for the processing of such materials. Because of its exemplary and fundamental character, the research is expected to provide a broad intellectual basis for the future design, processing and application of advanced functional nanomaterials. The interdisciplinary research approach will allow students to experience a broad education and to acquire a sound knowledge base in the area of polymer science and engineering. An exciting mix of fundamental science and applied research will place them at the cutting edge of today's workforce. One key element to integrated research and education is the formation of so-called Project Research Teams, which include high school students, undergraduate and graduate students, and faculty. This vehicle provides an exciting learning environment and creates teaching and mentoring opportunities for graduate students. All students will participate in outreach programs in collaboration with the Cleveland Museum of Natural History and the Shaker Heights School District. These activities will enhance the scientific and technological education of the public and reach out to a very diverse audience in the local community.
Cellulose nanocrystals (CNCs) are an intriguing class of nanomaterials that can be obtained from a wide variety of natural biosources. They have nanoscale size dimensions (5-30 nm diameter, 25-3000 nm long), a rod-like geometry, free hydroxyl groups exposed on its surface that can be readily chemically functionalized, and can be extracted from a range of abundantly available biorenewable feedstocks (trees, plants, tunicates, etc). Researchers at Case Western Reserve University in Cleveland, Ohio (CWRU) and the University of Fribourg, Switzerland (UF) have been investigating the use of these CNC as templates for a range of electro and photo-active species with the goal of developing new materials that hold promise in the area of flexible electronics and photovoltaic devices. They have shown that the CNCs can be used to template the self-assembly of a number of electroactive nanorod structures where metallic or semi-metallic nanoparticles or conducting polymers have been adsorbed on the CNC surface. These nanorods have allowed access to conducting foams and flexible mechanically stable films. The group has also shown that covalent functionalization of the CNCs is also possible as an alternative route to such electroactive nanorods. This research allowed students to gain an understanding of how a combination of chemistry, engineering, processing and biological resources/materials can be used to design and access new electroactive materials. Furthermore, the importance of communicating new science and cutting-edge technology to the public cannot be emphasized enough in order to inspire the development of the next generation of young scientists. As part of this program the CWRU team held a number of outreach activities at local elementary schools and at the Cleveland Museum of Natural History. At these events they used interactive demonstrations to highlight polymers, what they are, their properties and their applications in everyday activities. The program also fostered international interactions between students at CWRU and UF.
